Master Gardener: Earthworms good for soil, but think twice before moving them around

Q. I notice that my raised beds don't have any earthworms. Would it be a good idea to add some?

A. The answer to that one is somewhat complicated. Earthworms play many roles in our soils, most of them positive — they interact with fungi and bacteria, move literally mountains of soil, enriching it with their castings, improve water penetration and aeration, encourage mycorrhizae, and serve as food for other organisms.

That said, they can store pesticides and heavy metals in their bodies — up to 10 times as much as soil can hold — and so may poison birds and other creatures that eat them.

Also, our native worms quickly leave areas where the earth has been tilled or trees removed, and remain only in undisturbed oak savanna and other California habitats. So those in your yard are introduced species from Europe, Asia or Latin America, which thrive in disturbed environments — croplands, orchards, pastures, urban and suburban environments. And because few studies have been done on the role of earthworms in ecosystems, we just don't know what effects moving them around might have.